The present invention relates generally to a frequency detecting circuit. Particularly, the present invention relates to a frequency detecting circuit applied in a digital information reproducing system such as a digital audio disk system, which detects, in an information signal-demodulating mode, an inverting period, that is, the period during which the signal remains at a constant logic level without inverting, of the information signal stored in a recording medium so as to keep the inverting period value within minimum and maximum period limit values specially predetermined by any of some specific types of modulating techniques.
A pulse code modulation (PCM) system which converts an analog signal such as an audio signal to a digital signal has been recently developed, which takes advantage of the high quality of reproduction signals. For recording an audio signal in accordance with the PCM system, an analog signal is sampled, quantified, and coded to generate a binary signal which is then recorded on a data recording medium such as an optical disk, called a digital audio disk (DAD). At this time, after being encoded using error correction techniques, the signal is modulated by a predetermined modulation system. An eight to fourteen modulation (EFM) system, 3-position modulation (3PM) system or the like has been employed for this modulation, which uses predetermined maximum and minimum inverting periods.
For detecting the inverting period, it is necessary to obtain a frequency relative ratio of an input signal as a pulse signal and a demodulating clock signal. By convention, the frequency detection has been performed by matching phases of the input information signal and the demodulating clock signal. In this frequency detection, when the reception of the demodulating clock is temporarily stopped, the detection of the inverting period becomes impossible. This results in an unexact demodulation of the digital information signal.